High intensity focused ultrasonic energy (i.e., having a frequency greater than about 20 kilohertz), may be used therapeutically to treat internal tissue regions within a patient. For example, ultrasonic waves may be used to induce coagulation and/or necrosis in a target tissue region, such as a tumor. In this process, the ultrasonic energy is “absorbed” by the tissue, causing the generation of heat. The absorbed energy heats the targeted tissue cells to temperatures that exceed protein denaturation thresholds, usually above 60° C., resulting in coagulation and/or necrosis of the tissue.
During a focused ultrasound procedure, small gas bubbles, or “micro-bubbles,” may be generated in the liquid contained in the tissue, due to the stress resulting from negative pressure produced by the propagating ultrasonic waves and/or from when the heated liquid ruptures and is filled with gas/vapor. On the one hand, the micro-bubbles have a positive treatment effect by generating higher harmonic frequencies of the original wave energy, thereby increasing the absorption of energy in the tissue, and by multiple reflection that extends the acoustic pass in the target region. On the other hand, the reaction of tissue containing a higher relative percentage of micro-bubbles to the continued application of the ultrasound energy is non-linear and difficult to predict. For example, the micro-bubbles may collapse due to the applied stress from an acoustic field. This mechanism, called “cavitation,” may cause extensive tissue damage beyond that targeted, and may be difficult to control.